This invention relates to a counter-flow asphalt plant used to produce a variety of asphalt compositions. More specifically, this invention relates to a counter-flow asphalt plant having independently controlled drying and mixing sections to vary material residence times and mixing cycles to improve economy and efficiency of plant operations.
Several techniques and numerous equipment arrangements for the preparation of asphaltic cement, also referred by the trade as "hotmix" or "HMA", are known from the prior art. Particularly relevant to the present invention is the continuous production of asphalt compositions in a drum mixer asphalt plant. Typically, water-laden virgin aggregates are dried and heated within a rotating, open-ended drum mixer through radiant, convective and conductive heat transfer from a stream of hot gases produced by a burner flame. As the heated virgin aggregate flows through the drum mixer, it is combined with liquid asphalt and mineral binder to produce an asphaltic composition as the desired end-product. Optionally, prior to mixing the virgin aggregate and liquid asphalt, reclaimed or recycled asphalt pavement (RAP) may be added once it is crushed up or ground to a suitable size. The RAP is typically mixed with the heated virgin aggregate in the drum mixer at a point prior to adding the liquid asphalt and mineral fines.
The asphalt industry has traditionally faced many environmental challenges. The drum mixer characteristically generates, as by-products, a gaseous hydrocarbon emission (known as blue smoke) and sticky dust particles covered with asphalt. Early asphalt plants exposed the liquid asphalt or RAP material to excessive temperatures within the drum mixer or put the materials in close proximity with the burner flame which caused serious product degradation. Health and safety hazards resulted from the substantial air pollution control problems due to the blue-smoke produced when hydrocarbon constituents in the asphalt are driven off and released into the atmosphere. The exhaust gases of the asphalt plant are fed to air pollution control equipment, typically a baghouse. Within the baghouse, the blue-smoke condenses on the filter bags and the asphalt-covered dust particles stick to and plug-up the filter bags, thereby presenting a serious fire hazard and reducing filter efficiency and useful life. Significant investments and efforts were previously made by the industry in attempting to control blue-smoke emissions attributed to hydrocarbon volatile gases and particulates from both the liquid asphalt and recycle material.
The earlier environmental problems were further exacerbated by the processing technique standard in the industry which required the asphalt ingredients with the drum mixer to flow in the same direction (i.e., co-current flow) as the hot gases for heating and drying the aggregate. Thus, the asphalt component of recycle material and liquid asphalt itself came in direct contact with the hot gas stream and, in some instances, even the burner flame itself.
For limited production, another common processing technique in the industry was known as a batch plant. Briefly, a batch-plant dryer included a rotating cylindrical drum in which aggregate was fed to an inlet end and heated by a hot gas stream flowing through the drum in a direction opposite the aggregate. The hot aggregate was expelled from the drum to a bucket conveyor which elevated the aggregate to a batch tower where it was mixed with liquid asphalt, dumped into a truck and carried to the job site. In addition to significant environmental concerns, batch plants suffered from low production rates, operating inefficiencies and product storage problems.
One prior art modification of the foregoing plant operation applied to a continuous process included a dryer elevated above a mixer for gravity feed thereto. However, this type system required a separate blue smoke fan to vent the mixer back to the burner for the dryer and also required the necessary ductwork for containment of the mixer gases. A very serious drawback, however, was the cooling effect in the mixer associated with the blue smoke fan.
Many of the earlier problems experienced by asphalt plants were solved with the development of modem day counter-flow technology as disclosed in my earlier patent Hawkins U.S. Pat. No. 4,787,938 which is incorporated herein by reference and which was first commercially introduced by Standard Havens, Inc. in 1986. The asphalt industry began to standardize on the counter-flow processing technique in which the ingredients of the asphaltic composition and the hot gas stream flow through a single, rotating drum mixer in opposite directions. Combustion equipment extends into the drum mixer to generate the hot gas stream at an intermediate point within the drum mixer. Accordingly, the drum mixer includes three zones. From the end of the drum where the virgin aggregate feeds, the three zones include a drying/heating zone to dry and heat virgin aggregate, a combustion zone to generate a hot gas stream for the drying/heating zone, and a mixing zone to mix hot aggregate, recycle material and liquid asphalt to produce an asphaltic composition for discharge from the lower end of the drum mixer.
Not only did the counter-flow process with its three zones vastly improve heat transfer characteristics, more importantly it provided a process in which the liquid asphalt and recycle material were isolated from the burner flame and the hot gas stream generated by the combustion equipment. Counter-flow operation represented a solution to the vexing problem of blue-smoke and all the health and safety hazards associated with blue-smoke.
A more complete understanding of the early equipment and processing techniques used by the asphalt industry can be found in the extensive listing of prior art patents and printed publications contained in my earlier patents Hawkins U.S. Pat. No. 5,364,182 issued Nov. 15, 1994, Hawkins U.S. Pat. No. 5,470,146 issued Nov. 28, 1995, and Hawkins U.S. Pat. No. 5,664,881 issued Sep. 9, 1997. Indeed, as a result of my first patent Hawkins U.S. Pat. No. 4,787,938 becoming involved in protracted litigation, the prior art collection cited in the foregoing patents is thought to be a thorough and exhaustive bibliographic listing of asphalt technology and such prior art is specifically incorporated herein by reference.
With many of the health and safety issues associated with asphalt production solved by the advent of counter-flow technology, attention has now shifted to operational inefficiencies which are manifest as product failing to conform to specifications and as excessive production costs.
During startup and shutdown operations, modem day counter-flow plants experience substantial product waste which affect operating costs. When a counter-flow plant is first started, one to two truckloads of material is wasted. This results from the larger stones in the initial charge of feed aggregate moving through the drum mixer more quickly than the rest of the aggregate. Consequently, the liquid asphalt cannot be added until all sizes of stones have reached the mixing zone.
When shutdown of the counter-flow plant is required, the entire drum mixer must be emptied. Because of the difficulty in precisely separating the uncoated aggregate from the coated aggregate, the remnant load in the mixing zone frequently fails to satisfy product specifications and must be scrapped. Uncoated aggregate can, of course, be reused during subsequent operations but the energy costs associated with the first drying and heating of the material are lost and the material must be re-handled.
The recycle feed assembly represents special concern during shutdown, even for brief periods of time and as experienced with overnight interruption of operations. Fed at an intermediate point in the drum mixer, the recycle material is normally introduced to a stationary collar which encircles the drum mixer. Blades on the drum itself stir through the recycle material contained by the collar and cause it to fall through openings in the drum. Any material which reaches the bottom of the recycle collar is trapped since it is below the level of the drum itself. Thus plugging causes numerous problems including abrasive wear on the drum shell during continuous operations. During shutdown periods, however, severe sticking often results as a result of plug buildup in the recycle collar.
The recycle assembly itself may add additional costs to the plant due to layout considerations. Conventional counter-flow recycle collars must have the inlet on the uphill rotation of the drum mixer. They also discharge finished product on the uphill side of the drum mixer. Layout considerations generally require the recycle feeder to be on the downhill rotational side of the drum. Therefore, the recycle conveyor must extend up and over the drum itself in order to feed the collar on the uphill side.
As a result of the size and weight of the equipment during normal processing conditions, the drive rollers which rotate the drum mixer are characteristically large and, therefore, expensive both initially and as a maintenance item.
Since the counter-flow process is carried out in a single rotating vessel, control techniques are necessarily limited. Basically the plant operator can control feed rates, the relative proportion of the materials such as aggregate, recycle, asphalt and binder, and the amount of heat energy introduced to the process by the combustion equipment. However, adjustments to such parameters typically must occur after the end-product is produced and analyzed. If the product is off-spec then changes in operating conditions must be made, and the output is analyzed once again to see if the product then meets specifications. Parameters such as temperatures intermediate the drum mixer, mixing cycles and residence times of component materials simply cannot be controlled in prior art counter-flow plants.
A need remains in the industry for improved counter-flow asphalt plant design and operating techniques to address the problems and drawbacks heretofore experienced with modem counter-flow production. The primary objective of this invention is to meet this need.